The present invention relates generally to optical disk recording methods and apparatus for recording desired information on an optical disk, such as a CD-R, CD-RW, CD-WO, MD or DVD, by light power, and more particularly to a method of and device for controlling the light power to be used for recording on the optical disk.
In the conventionally known write-once and rewritable optical disks (CD-Rs and CD-RWs), data-carrying pits are formed by laser light, having controlled power levels as shown in FIG. 3, onto a disk surface coated with a dye-based recording material. In order to form such pits in the optical disks stably in constant conditions, it is necessary to control a laser-diode driving current in such a way that constant read and write laser power levels Pr and Pw are achieved at any time. Specifically, to this end, part of the laser light emitted from the laser diode is detected as an electric current by a monitor photo detector and the thus-detected current value is compared to a predetermined target value. ALPC (Automatic Laser Power Control) servo circuit is normally provided to control the laser diode output to always follow the target value. The ALPC servo circuit is generally composed of a detection-related component group including the above-mentioned monitor photo detector and a drive-related component group for driving the photo detector. The detection-related component group has inherent error factors such as a dark current in the photo detector and offsets in a current-to-voltage (I/V) converter amplifier and in sample and hole circuits, and the drive-related component group also has inherent error factors such as offsets in buffer amplifiers and in current amplifiers. Thus, it has been a common practice to compensate for sensitivity differentials in the detection-related component group and automatically calibrate the sensitivities and offsets of the drive-related component group, by measuring such electrical offsets, canceling offsets in input signals to A/D converters and also performing test laser light emission.
A series of these automatic calibration operations for the ALPC servo system is generally performed prior to a trial write procedure, commonly called "OPC" (Optimum Power Calibration), that is intended to determine an optimum recording power for each disk; specifically, it has been conventional to carry out such automatic calibration operations upon power-up or at one other time, for each linear recording speed, in combination with the OPC procedure.
Then, actual recording on the optical disk is performed using the optimum recording power determined through the OPC procedure; however, if the laser light power at the beginning of the recording is increased gradually from the zero level with the servo control activated, part of pit-forming regions would be undesirably left unrecorded. To avoid this inconvenience, there has been employed a scheme, in accordance with which the laser light is raised instantaneously up to the optimum recording power level with the servo loop opened and then the servo loop is closed once the optimum recording power level is reached.
FIG. 4 shows a typical example of relationship between a laser-diode driving current and a resultant light output from the laser diode. As shown, a variation occurs in this relationship as the temperature changes from a value T to another value T'. Assuming that the electric current levels to provide a read power level Pr and write power level Pw at the temperature T are Ir and Iw, these current levels have to be changed to Ir' and Iw', respectively, at the other temperature T', in order to keep the laser light output constant. Particularly, with the recent high-power lasers, the driving current has to be considerably varied to provide constant read and write power levels, because characteristics of these high-power lasers tend to be greatly varied due to temperature changes.
When the temperature changes during the write operations with the servo control active, substantially no problem is encountered because the servo control allows the driving current to automatically follow the light output so that the resultant light output remains constant in level. However, in case the temperature changes, for example, from T to T' during a recording standby state, the write power would amount only to a level Pw' even though the writing current level Iw, which was optimum at the last automatic calibration, is applied to the recording at the T' temperature. Writing current level Iw' must be applied, to obtain the target write power level Pw. Because an output value from a digital-to-analog converter (hereinafter also called a DAC value) for initiating the laser light emission, obtained through the last automatic calibration, would thus invite in an unwanted deviation in the laser power when a temperature change occurs during the recording-standby state, further automatic calibration operations are necessary to again determine an appropriate DAC value for initiating the laser light emission.
Such automatic calibration operations may be carried out upon lapse of every predetermined time, at predetermined time intervals, or in response to each temperature change detected within the device in question. But, these automatic calibration operations are initiated irrespective of a user's intention in any case, so that if reproduction of the recorded data is requested in the course of the calibration, the reproduction performance would be unavoidably degraded even though some preventive measure, such as temporary suspension of the calibration,. is taken. Such degradation of the reproduction performance would lead to a significant problem in reproducing CD-ROMs of video games and the like of which relatively quick responsiveness is required.
Further, detecting temperature changes within the device requires a separate temperature sensor, which would add to the costs and also cause detection errors unless it is located close to the laser diode. If such a temperature sensor is provided above the write head, the number of signal lines on the flexible circuit board has to be increased, which would further add to the costs and become a major cause of degradation in access performance.